Art. VIII. — On the Occurrence of Trachyte in Victoria. 



By EVELYN G. HOGG, M.A. 



[Eead 8th June, 1899.] 



The term " trachyte " appears to have been introduced by 

 Haiiy in 1822 to denote certain igneous rocks which present a 

 rough aspect in hand specimens, but since then the application of 

 the term has been much narrowed down and in the following 

 paper it is employed to describe a small group of igneous rocks 

 of interniediate basicity, in which the dominant felspar is the 

 sanidine variety of orthoclase, and in which the ground-mass, 

 usually holocrystalline, mainly consists of felspar microliths ; one 

 or more of the ferro-magnesian minerals — biotite, hornblende and 

 augite — may occur, while apatite, magnetite and zircon are often 

 present. In the normal trachyte free quartz is absent. 



Trachyte is of rare occurrence, and, though usually found in 

 rocks of fairly recent origin, is not confined to those rocks, as a 

 fresh augite-bearing trachyte is noted as occurring in rocks of 

 Lower Carboniferous age in the Garlton Hills, Haddingtonshire,^ 

 and a mica-trachyte of Permian age is reported from Copple- 

 stone, near Knowle Hill in Devonshire.- 



Owing to the interest which attaches to this rock, a brief notice 

 of its occurrence in Australia may not be out of place. 



Trachyte is noted in Queensland as occurring at Gladstone, 

 at the Glasshouse Mountains and in the Mackay district." The 

 author of this paper has had — through the kindness of Mr. A. G. 

 Maitland, F.G.S., formerly of the Queensland Geological Surv^ey, 

 and now Government Geologist of Western Australia — the 

 opportunity of studying specimens from the two last mentioned 

 localities. The trachyte from ^Nlount Coonowarin (Crookneck), 

 in the Glasshouse Mountains, is very similar in structure to that 



1 Hatch. Trans. Roy. Soc. E.lin. (1S92), xxxvii., pp. 115-126. 



2 Hatch. Geol. Matr. (1892), p. 250. 



« Jack and Etheridge. Geology and Palaeontology of Queensland and New Guinea, pp. 

 546, 714, 715, and 739. 



